autopkgtest logs should be served as text/plain

https://debusine.debian.net/artifact/38/ shows an example of a failed autopkgtest run. It has a number of files attached. testinfo.json is served with a reasonable Content-Type due to its extension, but all the others (log, regress-packages, regress-stderr, regress-stdout, summary, testbed-packages, and testpkg-version) are really text but are served as application/octet-stream. This makes them inconvenient to browse.

I don't think it makes sense to guess all these from their file names, since they're quite autopkgtest-specific. Perhaps we should add a nullable content_type column to File, and use that to allow code that uploads files to optionally override the Content-Type that will be used when serving them?